The present invention relates to an apparatus for discriminating a sheet transfer condition and, more particularly, to such an apparatus which detects failures in the transfer of sheets accurately and positively while the sheets are transferred by a pair of coactive rollers.
In a facsimile apparatus, copying machine or printer, for example, sheets such as original documents or recording sheets are pulled out one by one from a cassette or a tray and conveyed by a pair of registration rollers to a processing station which is a scanning station or a recording station. When two or more sheets are fed stacked together or when the thickness of a sheet is larger than a predetermined thickness, various troubles such as a sheet jam, sheet breakage or damage to the mechanism will be invited.
An implement heretofore known for avoiding such troubles comprises an additional pair of rollers located in a predetermined sheet feed path or utilizes the registration roller pair. With this known implement, a change in the gap between the rollers which is larger than a predetermined amount indicates the feed of two or more sheets stacked together. Sheet thickness is measured by a light source and a light receiving element which are located to face each other on the opposite sides of the sheet transfer path, so that an amount of light transmitted through a sheet represents a sheet thickness.
However, the rollers used for detecting the simultaneous feed of two or more sheets must have their outer peripheries treated with disproportionate accuracy to have predetermined diameters and be free from undulation. Also, the axis of rotations of the rollers need be held in a constant and stable relationship without any dislocation relative to each other, while the axis of rotation of one roller has to retract accurately from that of the other roller by an amount equal to a roller gap produced by a sheet which is being driven by the rollers. Admitting that all such requirements are fulfilled, accurate measurement of the roller gap or sheet thickness tends to fail after a long time of use of the rollers. Meanwhile, the light receiving element coactive with the light source cannot operate accurately or stably because deposition of dust or particles produced from sheets would affect the amount of light incident on the light receiving element. To overcome this problem, use must be made of a light source which has a relatively large output at the sacrifice of the economy.